Integrated circuits typically operate with power supplies of 5 volts or less and often must drive signals of a particular voltage level on-chip or off-chip. Merely as an example, an integrated circuit pre-amplifier may have a plurality of driver circuits for driving signals off-chip. For instance, an eight bit amplifier for driving eight signals off-chip might have eight driver circuits having an output stage like the output stage 101 shown in FIG. 1 for driving an off-hip load 102 through an output pad 104 of the integrated circuit. FIG. 1 shows only the output stage of the driver circuit in detail. The input signal source, VIN, that is to be driven onto the load 102 is supplied to one input terminal of an operational amplifier 105.
In the output stage 101, an output transistor M17 has its source coupled to a voltage rail 113, in this case 5 volts, and its drain coupled to node 107. Its gate is coupled to the output of the operational amplifier 105. Transistor M16 has its source coupled to the voltage rail 113, and its drain and gate coupled together to the gate of output transistor M17 and the output of the operational amplifier 105. Transistors M16 and M17 in this circuit are configured as a current mirror that essentially delivers current controlled by the operational amplifier 105 to the load. The input signal VIN is supplied to one input terminal of the operational amplifier and the other input terminal is coupled to the junction 110 of voltage divider 109 comprising resistors R0 and R1. Since an operational amplifier operates to drive the voltages at its two inputs to the same voltage, operational amplifier 105 drives the junction 110 between resistors R0 and R1 to VIN. The voltage at the output pad 104 is dependent on the input voltage, VIN, and the ratio of resistors R0 and R1. Specifically, with this configuration, the output voltage on pad 104 is ((R0+R1)/R1)*VIN. The current through the load 102 is dictated by the voltage placed on pad 104 and the resistance, Rext, of the load 102. This type of architecture is efficient in that it generates maximum output voltage because the only voltage drop from the rail is the Vds of M17. So the output voltage can go to a maximum value of VCC−VdsM17.
Transistor M15 has its current flow terminals (source and drain) coupled between the drain of output transistor M17 and the load 102. The source is coupled to the drain of transistor M17 at node 107 and the drain is coupled to the output node 104. A voltage divider 109 is coupled between the output node 104 and ground with the divided voltage supplied to the second input of the operational amplifier 105. Transistor M15 acts as a source follower at lower output voltages, preventing the Vds breakdown of transistor M17. At higher output voltages, transistor M15 acts as a pass gate, whereby the output voltage on node 104 follows the voltage at node 107 between transistors M15 and M17. This driver circuit should produce a very good output voltage range of about 0 to 4.5 volts.
In a multi-bit preamplifier circuit, (8-bit, for example) a single, “selected” driver typically drives the load over the full output range (e.g., about 0-4.5 volts), while the seven remaining, “unselected” drivers only need to drive their external loads to very low voltages (e.g., 0-1.5 volts). In such conditions, most of the excess voltage from the various drivers is dropped inside the chip. For example, if the unselected loads are to be driven to only 1 volt, then Vcc (5 volts)−1 volt=4 volts will be dropped inside the chip for each of the 7 unselected drivers. With seven drivers dumping 4 volts each on-chip, power dissipation on-chip can be quite substantial.
In many situations, e.g., when such circuits are employed in battery-powered devices, such as cellular telephones, PDAs (Personal Digital Assistants), and portable digital audio or video recording and playing devices, it is particularly desirable to minimize wasted power.